1. Technical Field
The present invention relates to a technique of modifying quality of an insulating film in a non-volatile memory element.
2. Related Art
In a non-volatile memory element configured as having a floating gate and a control gate disposed while placing an interpoly insulating film in between, data writing is accomplished when hot electrons generated in a channel region on a substrate are incorporated into the floating gate. Data is erased when the electrons are drawn out from the floating gate into the control gate based on the F-N (Fowler-Nordheim) tunneling phenomenon.
The non-volatile memory element having the interpoly insulating film between the floating gate and the control gate as described in the above has been known to suffer from a problem in that repetitive writing and erasure result in generation of electron traps or trap sites in the insulating film, and thereby degrades the durability. The insulating film is generally formed by the CVD process, and intrinsically contains a large amount of traps in the film. The film also has a nature of producing a large amount of traps by the repetitive writing and erasure.
Japanese Laid-Open Patent Publication No. H11-40681 discloses a technique by which a CVD oxide film is subjected to nitriding in an atmosphere containing NO or N2O so as to introduce therein nitrogen, to thereby improve the film quality. The above patent publication described that the modification of the film quality made it possible to improve the quality of the tunnel oxide film, to reduce ratio of electrons emitted from the floating gate and trapped by the trap sites in the tunnel oxide film during the erasure operation, to prevent the erasure efficiency from degrading, and to thereby succeed in obtaining a long-life, non-volatile memory element.